This invention relates in general to certain new and useful improvements in vocoder systems and, more particularly, to vocoder systems which provide wave form analysis and synthesis using Fourier transform representative signals.
In recent years, vocoder systems which are used for voice transmission and reception have received widespread attention. One of the more successful approaches to speech band width compression resides in the channel vocoder as described in U.S. Pat. No. 2,151,091 for Signal Transmission. This channel vocoder has evolved into an instrument of excellent intelligibility and good reliability. However, vocoders of this type have not found an extensive application due to the very substantial size of these instruments and the cost of purchase and maintenance.
One of the important criteria of an effective vocoder is that it must be capable of reproducing natural-sounding speech and retaining the voice individuality of the original speaker.
The present-day voice-excited vocoder generally uses one band pass filter (baseband filter) covering an approximate 250-930 Hz region for selecting a portion of the input voice signal to be transmitted with high fidelity together with several other band pass filters covering a frequency range of approximately 930 to 3230 Hz region in approximate 230 Hz increments, which are to be transmitted with lesser fidelity. The output of the baseband filter is immediately digitized while the outputs of all the other band pass filters are first rectified, smoothed, and then digitized. Thereafter, all of the channels are multiplexed for transmission.
At the receiving end, these channels are demultiplexed and converted back to analog form. The base band signal is then rectified and differentiated by an excitation generator and then passed through a bank of band pass filters and limiters to form a spectrally flattened excitation function. These excitation signals are modulated by the low frequency signals derived from the spectrum analysis process performed in the vocoder analyzer. The outputs from the modulators are then filtered to remove the nonlinear distortion introduced by the limiters and the resulting signals are combined with the base band signal to form the synthesized speech signal. This system, while it is effective, is nevertheless generally based upon analysis and synthesis of signals in analog format. There have been several attempts to use an all-digital approach to analysis and synthesis based on increased development of integrated circuit technology. However, these attempts to utilize a digital type system have not been particularly efficient from the standpoint of the utilization of large-scale integrated circuits.
The present invention provides a vocoder which permits a wave form analysis and synthesis technique based on Fourier transform representations of signals. In one embodiment of the present invention, a purely digital system approach is used. In another embodiment of the present invention, a hybrid approach using both analog and digital techniques is employed, and which has at least the same flexibility as the pure digital approach. In addition, the vocoder systems of the present invention are capable of handling wider band width signals and are less complex than the various vocoder systems heretofore designed.
It is therefore the primary object of the present invention to provide a vocoder system including a vocoder analyzer and vocoder synthesizer which is capable of reproducing natural-sounding speech and which retains the voice individuality of the original speech.
It is another object of the present invention to provide a system for wave form analysis and synthesis using Fourier transform representative signals.
It is a further object of the present invention to provide a vocoder system of the type stated which is capable of analyzing and synthesizing signals having a broad frequency spectrum.
It is another salient object of the present invention to provide a method of analyzing and synthesizing wave forms which is relatively economical and requires a minimum of operations.
It is an additional object of the present invention to provide a vocoder system of the type stated which is highly reliable and can be manufactured at a relatively low unit cost and which is nevertheless compact in size.
With the above and other objects in view, my invention reside in the novel features of form, construction, arrangement, and combination of parts presently described and pointed out in the claims